Miniaturization and weight reduction have recently required for electric/electronic devices such as cell phones, laptop computers and camcorders. For example, lithium secondary batteries, which have small size, high performances and large capacity and are chargeable/dischargeable, have been put to practical use.
The lithium secondary battery is not only used as a power source for small electronic devices, but also has a wide range of applications in electric vehicles, electric bicycles and the like, thus requiring more excellent high-temperature storage characteristics and lifetime characteristics than before. Particularly, in the case of the lithium secondary battery for electric vehicles, further improvement in stability is required.
Electronic components applied to the electric/electronic devices are required to maintain durability under high-voltage and high-temperature operating conditions and to maintain required electric or dielectric characteristics.
Recently, attempts have been made to employ various organic/inorganic materials in order to improve the heat resistance and high-voltage driving characteristics of electronic components. For example, as the material of a functional layer such as a protective membrane and a heat radiation layer of components, inorganic oxide materials such as alumina (Al2O3) are employed.
For example, a secondary battery capable of supplying a driving voltage to various electric/electronic devices without connecting an additional power source is employed. The secondary battery can be repeatedly charged and discharged, and lithium ion secondary batteries have been actively developed, recently.
Generally, the secondary battery includes an electrode assembly in which a positive electrode and a negative electrode are disposed via a separator, and the positive electrode and the negative electrode can be formed by applying an electrode active material on an electrode current collector. The separator can be formed of a polymer film made of a polyethylene-based resin.
When charging and discharging are repeated under high-voltage and high-temperature conditions in the secondary battery, the electrode characteristics deteriorate, thus making it difficult to maintain a desired capacity for a long time, and short circuit may occur between the electrodes due to shrinkage of the separator which is a polymer film material. Therefore, an alumina coating layer has recently been utilized as a protective membrane for electrodes.
KR 10-1511935 B (Patent Document 1) refers to an electrode assembly in which alumina is coated on a surface of a lithium cobalt-based oxide, but discloses neither conditions for formation of a uniform thin alumina coating layer nor development of the material.
To achieve higher capacity/higher energy density of the secondary battery, there is a need to reduce the thickness of an inorganic oxide porous membrane and a separator which do not contribute to the battery capacity. For example, JP 2010-205719 A (Patent Document 2) proposes a technique for forming an inorganic oxide porous membrane containing an inorganic oxide filler having insulating properties on a surface of a positive electrode, a negative electrode or a separator that constitutes a lithium ion secondary battery. It is considered that the inorganic oxide porous membrane has high heat resistance and can suppress abrupt contraction of the separator. However, there was a problem that, even when the inorganic oxide porous membrane is formed using inorganic oxide powders satisfying various physical properties mentioned in these patent documents, it is difficult to reduce the thickness of the thus obtained inorganic oxide porous membrane and to impart the heat resistance to the separator.